1. Field of the Invention
The present invention relates to a hybrid driving apparatus including, in its power transmission path, a transmission mechanism with a plurality of selectable transmission stages such as a transfer device, as well as to a method of controlling the same.
2. Description of the Background Art
Recently, a hybrid vehicle mounting a hybrid driving apparatus having an engine and an electric motor as power sources has come to be practically used to attain higher fuel consumption efficiency. In such a hybrid vehicle, an electric motor and/or a generator mechanically coupled to the engine is appropriately controlled such that engine operation is adjusted to an optimal operation point, thereby to attain improved fuel consumption efficiency. Specifically, when viewed from the engine, the electric motor and/or generator constitutes a kind of continuously variable transmission (CVT). As a result, the engine can operate while maintaining one same operation point, independent of the speed of hybrid vehicle.
Conventionally, a vehicle mounted with a transfer device has been known, which allows switching to a low-speed side transmission stage in accordance with driving operation, to coop with a situation requiring larger driving torque such as traveling on rough road or towing. By way of example, Japanese Patent Laying-Open No. 10-250395 discloses a transfer device in which a changeover mechanism is set to a low-speed side by an operation of a transfer selector lever from high speed to low speed position, so that speed of rotation of an output shaft of transmission is reduced and transmitted for driving to the front and rear wheels, through a planetary gear mechanism.
By installing such a transfer device in the hybrid vehicle described above, higher driving torque can be generated. Here, if the transfer device is switched to the low-speed side, that is, to a larger transmission gear ratio, speed of rotation of the electric motor and the like increases because of the switching in transmission gear ratio, even if the vehicle travels at the same speed. On the other hand, the optimal operation point of the engine is maintained regardless of the switching of transfer device.
Therefore, even if the transfer device is switched to the low-speed side, engine sound or vibration associated with the engine operation hardly increases. As a result, it might be often the case that the driver is unaware that the vehicle is traveling with the low-speed side gear stage. It is possible that driving continues for a long period of time while the transfer device is kept set to the low-speed side transmission stage. It is concerned that the speed of rotation of electric motor might exceed tolerable rotation speed, or there might be excessive load on the electric motor or a power converter supplying electric power to the electric motor.